Two-stroke conventional internal combustion engines with carburetors are used in many different areas. One is in chainsaws, which are commonly used outside in forest working characterized by a large variation in climate. The engine therefore has for instance to manage to be run at high speed, in cold climate and in rain. In such use the functionality of the carburetor is very important. It has to provide the right amount of fuel to the engine in relation to different conditions. The fuel/air ratio is important for the operation of the engine, and depends on temperature, pressure, engine speed and load. The carburetor is therefore calibrated at manufacturing to be able to provide, at the engines operating point, the right amount of fuel and air in order for the engine to operate properly.
The operating point is related to operation where the engine has reached its operating temperature. The carburetors calibration is based on such an operating state. On the other hand, when the engine is cold and about to be started, the calibration will not be able provide sufficient conditions for that. Therefore the carburetor is equipped with a choke to increase the fuel ratio in the engine to enable it to start. The fuel/air mixture is enriched.
The invention concerns the kind of carburetors where engaging the choke also affects the throttle valve to open somewhat providing a starting throttle. Thus the normal starting position is a closed choke valve and a slightly opened throttle valve.
In many carburetors the choke valve and the throttle valve have one respective lever which can be interlocked during the start of the engine providing starting position of the throttle valve and the choke valve. The choke valve lever is controlled in one rotational direction by a choke valve conveyor, and the choke valve axle can be held in two detent positions, a first detent position of closed choke valve and a second detent position of open choke valve. This is often implemented by having a spring pressing a ball towards a suitably placed bowl formed notched on the choke valve axle, one notch for the first detent position and a second notch for the second detent position. During normal engine running the choke valve is held stable in the second detent position of opened choke valve and at start the choke valve is normally held in the first detent position. However, in many situations it is desirable to start the engine without choke, i.e. the second detent position, but with a start throttle. When the throttle wire is activated after the start of the engine the interlock between the throttle valve lever and the choke valve lever is released. Often both the throttle valve lever and the choke valve lever are spring loaded towards opened choke valve respective closed throttle valve. Thus when the interlock is released the choke valve spring acts to open the choke valve, however, the choke valve spring must overcome the friction of the first detent position to move the choke valve from closed to opened. If this friction is not overcome the choke valve remains closed after the throttle wire is activated, which is undesirable.
One problem with these conventional manual chokes is that its functionality is very much related to the engines temperature at start. During a warmer climate, for instance above zero degrees Celsius, the engine needs less fuel in order to start. The needed fuel/air enrichment for the engine to start goes down when the temperature goes up. Despite the temperature variations at use, the choke is designed to provide a maximum fuel/air ratio that is needed at a very low temperature.
When the worker pulls the starting cord he/she has to recognize that the engine ignites. Every new pull will increase the enrichment in the engine and if the worker does not deactivate the choke after ignition, the enrichment will reach such a high level that the engine cannot start. The higher the temperature, the bigger the risk this will happen. An object of the present invention is therefore to provide a choke for a carburetor internal combustion engine, which is designed to consider the variations in climate where the engine is used.
Further there is a demand to have chain saws where the two separate motions are required to set the start position and it is an object of the invention to present a choke actuator needing two separate motions to arrive at the start position of slightly opened throttle valve and closed choke valve.
Another object of the invention is to provide a low friction arrangement for the first detent function. And further to provide a simplified implementation of the detent positions.